In a web-fed printing press, a web of material, typically paper, is fed from a storage mechanism, such as a reel stand, to one or more printing units which imprint the web with images (signatures). The imprinted web is then typically driven through respective processing units such as a dryer unit, coding equipment, and a cutting apparatus. The web-fed system allows for high volume production of signatures from each printing plate because of the rate, for example 1,700 feet per minute, at which the web moves past the printing cylinder.
Printing cylinders are typically approximately 71/4 inches in diameter, having a circumference of approximately 223/4 inches. The signature image is produced on the surface of an aluminum printing plate, the printing plate being mounted about the circumference of the print cylinder.
As the web travels through the press, it comes into contact with the printing plate as the printing cylinder rotates. The printing plate imprints an image onto the web corresponding to the printing plate's image surface. The image surface of the plate comprises a highly ordered matrix of small dots, typically on the order of 10,000-15,000 per square inch. The image printed on the web is set off by a background, corresponding to the nonprinted regions of the image. This background corresponds to the nonimage surface of the printing plate. To maximize print quality, it is imperative that the nonimage surface of the plate be kept clean so that the printed image is set off against a clean background.
As the continuous web travels through various printing stages, the signatures are inspected for printing flaws. For example, a crack or scratch in the nonimage surface of the plate may accumulate ink, resulting in a marred printed page. In addition, dust or debris from the web or the environment may become lodged between the dots, resulting in a defective printed image. A defective plate produces a phenomenon called "stippling", or a marring of the printed page. When stippling is detected, it is necessary to correct the problem immediately, which entails shutting down the entire web-fed press operation. This disrupts time schedules, which are extremely important in the context of daily or weekly news publications, for example. Moreover, down-time results in considerable wastage due to the high speed of operation. Thus, while occasional stippling is unavoidable, it is desirable to correct the problem and resume press operation as quickly as possible.
The manner in which a marred printing plate is repaired depends on the seriousness of the flaw. A major defect in the print plate often requires removal of the plate from the cylinder and repair, reconstruction or replacement of the plate. For minor adjustments, such as removal of extraneous particles from the plate surface, lithographers often perform "on-cylinder" repair. It is desirable, to the extent possible, to repair the plate without removing it from the cylinder, thus minimizing press down-time.
Presently known methods of print plate repair typically require two persons working simultaneously. Because of the fine detail work involved, the immediate stippling region must be illuminated. One person typically directs illumination to the work site, while another person performs the repair. When performing on-cylinder repair of a printing plate, it is essential that the plate not be bumped, scratched, dented or marred in any way. This makes it difficult to attach illumination or magnification equipment to the cylinder surface. Consequently, lithographers typically attempt to hold a magnifying lens in one hand and a stipple in the other hand. A stipple is a pencil-like instrument having a sharp, pointed extension which is used to cross-hatch between the dots to clean the surface of the print plate. Much skill and dexterity is involved, as the dots are quite small and very close together. The lithographer must repair the stippling, as quickly as possible, being careful not to scratch or in any way impair the surface of the printing plate, and at the same time illuminate and magnify the stippled region. In addition, because of the inherently limited space available in a press room, it is difficult to accommodate more than one person at a time.